ok, wot u do is this...

First off, if you or your mates get a chance to see it(and providing theyre wearing them) try to get the number of the officer(s) who assaulted you. They are usually worn on the lapel.

Get your injuries photographed, and examined by a doctor. Get the doctor to sign a statement to the effect that your injuries, in his/her opinion are consistant with being attacked in the way you describe.

Then, get eyewitnesses. Make a request on indimedia if you have to. Get them to write and sign statements as to what they saw, and make sure they are willing to testify.

Collect any photographic evidence. Get copies of any pictures taken of the incedent, and importantly suss out if there was any cctv cameras rolling, or if you were bein filmed by cops. Mark Thomas's website has a whole lot of info on how to request this stuff, its your right and can be used as evidence against the cops. Do it now!

Then, get a solicitor. Go to Moss & Co, Bindmans, or Bhatt Murphy - you should find em in yellow pages. They are used to dealing with this stuff. They will recommend other solicitors, if they cant do it themselves.

Then, you could make an official police complaint, information on how you do this is on the Mets own website but wot I recomend is that you sue those coppers sorry big fat asses!

Good luck!